Free radical polymerization is a preferred technique in the synthesis of many polymers. One drawback of free radical polymerization is the lack of control over the polymer architecture produced. The type and amount of initiator, temperature, and delayed monomer feeds have all been used to control the final structure and size of the polymer particles.
Living polymers offer some control of the polymer structure. Living polymers are polymers having an active/growing end on a polymer chain (non-terminated polymer chain). Most commonly, living ends are formed by anionic polymerization in non-polar solvent, or involve a capping-mechanism to stop a growing free radical, then restarting the polymer growth by reversible removal of the cap.
Chemical trapping agents, polymerization mediators, or polymerization control agents are known in the art as a means of capping, or coupling with, a living polymer radical by a reversible means, then uncapping the living radical for latter continued polymerization. The capping, coupling or control mechanism could be reversible termination, chain transfer, complexation, the use of stable free radical forming agents, or other such chemical means that can be used to control radical activity. Unfortunately such a method requires additional purification costs to remove the trapping agent from the product. The trapping agents also add kinetic limitations, dependent on the specific trapping agent used.
U.S. Pat. No. 5,173,551 and “Studies of the Polymerization of Methacrylic Acid via Free-Radical Retrograde Precipitation Polymerization Process”, J. Applied Polymer Science, Vol. 62, 2039-2051 (1996) describe the use of a free-radical retrograde polymerization process as a means of controlling the polymer molecular weight distribution. In this process a monomer mixture in a solvent is initiated by a solvent-soluble free radical initiator to produce polymer radicals that precipitate into polymer-rich phases in a solvent. The above patent also addresses the capability of FRRPP to control the activity of polymer radicals and use them to produce copolymers. Additionally it provides a detailed description of the original process, a possible apparatus for running it, and ways of regulating the rate of reaction through changing temperature and adding or removing monomers or inert diluents.
U.S. patent application Ser. Nos. 10/045,881 and 10/045,725 describe free radical dispersions and copolymers that can be made by free radical retrograde precipitation. One drawback of FRRPP is that it requires poor solvents that precipitate a polymer above the lower critical solution temperature.
There is a need for a method to control polymer architecture, without the drawbacks found in the current art.
Surprisingly it has been found that a stable dispersion of particles having an average of one or more than one living radical per particle can be produced without chemical capping agents or mediation, and without the need to produce said radicals and particles through a free radical retrograde precipitation polymerization process.